Such an apparatus may comprise the following known elements:
conveyance rollers to bear and to carry along the products to be heated according to a longitudinal and notably horizontal direction; these rollers coming one after the other, according to this longitudinal direction and rotating around shafts which are parallel to a transverse and notably horizontal direction, perpendicular to this longitudinal direction; and wherein they are supported by end bearings,
induction coils supplied with electric current to generate a periodically variable magnetic flux,
and a looped magnetic circuit constituted at its top section by high magnetic conductivity elements and by, at its bottom section, the product to be heated to channel this flux by forming flux loops which pass twice upwards and downwards through the product to be heated, and which close longitudinally, on top of and under this product,
the rollers being made of composite material and including each a magnetic stack and a stiffening element; the magnetic stack taking at least half of the roller volume and being made of high magnetic conductivity elements stacked according to the roller axis so that the roller makes out part of the said looped magnetic circuit; the stiffening element being metallic, resists to at least the tension strains and extends, according to the said transverse direction, on the whole roller length inside the magnetic stacking to that the roller resists to the bending strains generated by the weight of the product to be heated.
A first known apparatus including such elements, is described in the U.S. Pat. No. 3,008,026 (Kennedy). According to this patent, the magnetic stack of each conveyance roller is constituted by thick removable disks which may have different magnetic conductivity values to allow suitable distribution of the heating magnetic flux according to the thickness of the product to be heated. The rollers are arranged by pairs above and under the product to be heated and their magnetic disks are in contact with this product. The induction coils surround the rollers, above and under the product to be heated, in order to ensure symmetric flux distribution. These rollers are in contact with the product to prevent any vertical movement due to the magnetic forces created by the heating flux.
As will be understood by those skilled in this art, this first known apparatus is exclusively designed for heating of a thin strip at a low temperature which is likely to be under 500.degree. C., even if this patent mentions heat treating and forming as possible applications of the invention. As a matter of fact, the heating power seems rather weak because of the coil arrangement and because a high flux would raise the temperature of the magnetic disks beyond their Curie point, thus entailing loss of their magnetic conductivity. Such a flux would also heat indirectly the shafts which make out the stiffening elements of the rollers, since these shafts are normally of steel presenting a sensitive magnetic conductivity and would therefore be crossed by such flux. By such heating, these shafts would loose part of their mechanical properties. Heating of these shafts and disks is more important as no thermal insulation is provided between the product to be heated and the conveyance rollers. At last, maintenance of such an apparatus would be costly since there are coils under a very hot product which might entail fall of hot fragments such as oxidation flakes.
In view of the above, the Kennedy patent, for those skilled in the art, does not seem to give useful indications for cases where heavy and thick products are to be heated at high temperature.
On the contrary, the present invention applies to cases where thick metal products are to be heated or warmed up to a high temperature, in order, for example, to facilitate further distortion. It more specially applies to cases where these products are long steel industry products, as for example, flat steel products, which are still hot during rolling procedures, and which have to be heated up to a temperature of about 1000.degree. to 1200.degree. C. to allow continuation of the rolling process in good conditions. The thickness of such products can, for instance, be included between about 25 and 250 mm, and the power which must be dissipated to heat such products can be included between about 10 and 200 W/cm1. This dissipation results from the fact that the product is crossed by the variable magnetic flux generated by an inductor and that this product is electrically conductive. Since its temperature is above the Curie point, which varies according to the alloys while always remaining under 770.degree. C., the product is prevented from being ferromagnetic. However the products could sometimes be aluminium plates or other nonmagnetic metals to be held at the correct rolling temperature.
It is precised that heating of such products can be obtained by a flux passing through the smaller dimension or thickness of the flat product. For what concerns the necessary flux variation, it can be obtained by period variation, for example, sine-shaped, of an inductive current in stationary coils. It can also be obtained by longitudinal or transverse displacement of drift field waves generated by a stationary multiphase inductor. It can also result from periodic reluctance variation of a DC-energized magnetic circuit, or by mechanical displacement of DC-energized fields.
For known industrial apparatus using roller conveyors associated with high power and high induction power heating components, these components are installed between the rollers so that the latter are as far as possible from the variable fluxes and so that they are not heated. There is no intermediate support plate between the rollers to bear the product to be heated. This is for example, the case of the apparatus as for the English Pat. No. 1 453 483 filed Mar. 7, 1974 (the Electricity Council, inventor: Ralph Waggott) or the apparatus as for the U.S. Pat. No. 3,471,673, inventor: Harold Grote Frostick.
These second and third known apparatus have the disadvantage of the heating power supplied by the inductors which is limited by the confined space available between the conveyance rollers or near the latter for passage of the variable magnetic flux. If such flux is vertical, this power is often notably weak because of the lack of intermediate support plates. This allows only small intervals between rollers if the temperature of the product to be heated reduces its bending strength between rollers.
It was proposed to increase the heating power of these second and third known apparatus by enlarging their length according to the conveyance direction. However this does not only increase the cost of the apparatus but also that of the buildings which have them. In addition the thermal losses are increased and it costs more to reach the temperatures required for the product to be heated.
It was also proposed, in order to increase the heating power without enlarging the length of the heating apparatus, to augment the frequency of the magnetic flux variation. Indeed it is known that the electromotive forces induced inside the material to be heated are proportional to such frequency and that, for a flux variation amplitude unchanged at all points, the power dissipated in such material increases as the square of the frequency. However the search for increased frequency is limited by the fact that the variable flux only penetrates on a restricted thickness of the material to be heated and that such thickness decreases as the frequency increases. Moreover, the use of a high frequency creates, above all, important losses in the magnetic circuit of the inductor and makes necessary the use of a poor efficiency current generator. Therefore the cost of the power supplied to the material to be heated, embarassingly increases.
Besides, the cost for construction of the known apparatus is increased by the fact that the magnetic circuit must strictly channel the fluxes in predetermined intervals.